In general, a pattern having a plurality of fine factors is embedded in a printed matter, in order to prevent falsification or unauthorized use. The unauthorized use is detected in accordance with information, which is produced by the pattern. Furthermore, information (embedding information) for preventing the unauthorized use is embedded in map data, in order to prevent the unauthorized use with respect to the map data described in vector form (vector type). In two or three dimensional map data described in the vector type, the embedding information is embedded in polygons, which form a plane. For example, a method is known in which the map data is composed of aggregation of triangle polygons each of which is divided into four triangles. The electronic watermark data are embedded as embedding data in a triangle formed between the triangle polygons (a triangle which does not include vertexes of each triangle polygon). By using the above-mentioned method, it is difficult to remove the electronic watermark data without affecting the map data described in the vector type.
On the other hand, disclosure is made in Japanese Patent Publication 2001-160897 A about embedding the embedding information for the unauthorized use prevention in the map data described in the vector type. In the publication, information having an embedding standard layer and an embedding reference layer is inputted to map pictorial information representative of a coordinate sequence of layout points in each object. The information having the embedding standard layer and the embedding reference layer is inputted to layer information which is for use in managing a type of each object. An embedding reference object pair is selected which exists in a region having the same meaning and whose region does not have other objects. An object is selected in which it is difficult to find out its embedded location, with respect to the embedding reference object. Renewal embedding object information is produced in accordance with an existence location and/or a shape characteristic of the selected object and a renewal object is embedded on the basis of an embedding density in synchronism with the object information.
Recently, the surface of earth (ground level) is measured by using so-called laser three-dimensional measurement in order to obtain data of ground level as an original random point group data and to obtain map data in accordance with the original random point group data. More particularly, an aircraft irradiates a laser pulse beam towards the ground in order to obtain spatial coordinates of the ground level. In this event, the spatial position of the aircraft is calculated by using a GPS reference station positioned on the ground and a GPS receiver installed on the aircraft. The attitude of the aircraft is calculated by using a three-axis gyroscope.
Incidentally, the ground coordinates of each one pulse is produced as x, y, and z coordinates of laser beam reflected point by the irradiating angle of the laser mirror and the slant distance of the laser mirror, in accordance with the spatial position and the attitude of the aircraft that are obtained in the manner described above.
Inasmuch as the ground coordinates obtained in the manner described above is merely representative of the random point group data of the ground level, it is necessary to process the random point group data into the map data.
Inasmuch as the original point group data described above are point group data, which are merely dispersed spatially, the original point group data, does not have relationships among one another and does not have attributes, respectively. In other wards, the original point group data are merely representative of x, y, and z coordinates. Accordingly, it is impossible to use the method of embedding the electronic watermark data into the above-mentioned vector type map data, with respect to the original point group data. As a result, it is easy to produce the map data by the unauthorized use of the original random point group data.
In addition, it is difficult to reproduce the original random point group data in case of embedding the electronic watermark data into the original random point group data in accordance with random numbers, within a predetermined accuracy. Furthermore, it is difficult to prevent the unauthorized use in case of partially thinning out the original random point group data, in order to embed the electronic watermark data into the original random point group data.